Update Condition Monitor subscription API to remove preconditions.

Preconditions are currently used internally by the Monitor whenever a
set of preconditions are specified in the Monitor constructor. However,
preconditions are confusing when used as part of building a
Subscription. Currently, these APIs are not used and we recently removed
the single usage because it was the cause of a bug.

Test: atest ConditionMonitorTest
Bug: 278875229
Change-Id: I0de87f24cd826536e47207e1c44fae44608e45cd